The present invention relates to chemical vapor deposition, integrated circuit fabrication and, in particular, to methods for depositing transition metal-based layers on a dielectric substrate.
The semiconductor industry is committed to introducing copper interconnects as a replacement for conventional aluminum and aluminum alloy interconnects in future generations of semiconductor devices. With its greater current carrying capacity, the introduction of copper interconnects should reduce device geometry, power consumption and heat generation. However, copper is a fast diffuser in silicon and drifts in dielectrics, resulting in a deterioration of devices at low temperatures. To avoid unwanted migration of copper atoms, a barrier layer of a transition metal-based material, such as a tantalum-based material and more particularly tantalum or tantalum nitride, is typically used as a diffusion barrier between a copper interconnect layer and an underlying dielectric layer, such as a layer of silicon oxide. One method of providing the diffusion barrier is physical vapor deposition (PVD) by sputtering. However, sputter deposition, among other problems, cannot adequately cover the sidewalls of near-surface features having a high aspect ratio because sputtering is essentially a line-of-sight deposition process.
Two chemical vapor deposition (CVD) processes, thermal CVD and plasma-enhanced CVD (PECVD), are candidates to replace PVD and provide highly uniform layers that conform to topographical features having high aspect ratios. Thermal CVD is a high temperature process in which a flow of gaseous reactants over a heated semiconductor substrate chemically react to deposit a solid layer on the heated substrate. Plasma-enhanced CVD is a relatively low-temperature process which introduces a plasma to activate the gaseous reactants.
To deposit a transition metal-based barrier layer, both CVD processes react a vapor-phase reactant, for example a transition metal halide reagent, such as a tantalum halide or more particularly tantalum pentafluoride (TaF5), with a reducing gas, for example a hydrogen-containing gas, such as either hydrogen (H2) or ammonia (NH3). If, for example, the reducing gas is hydrogen and the vapor-phase reactant is a tantalum halide, tantalum (Ta) is deposited, while tantalum nitride (TaNx) is deposited if the reducing gas is a nitrogen-containing gas, such as ammonia or a mixture of nitrogen and hydrogen. The chemical reduction of the transition metal halide vapor-phase reactant produces halogen atoms as a by-product.
The layer of transition metal-based material deposited by either of the CVD methods using a gas mixture comprising a transition metal halide vapor-phase reactant will incorporate a low residual level of the by-product halogen atoms as an unwanted impurity. For example, a layer of tantalum deposited on a substrate by plasma-enhanced CVD using, for example, tantalum pentafluoride will usually contain about 0.5 atomic percent of the residual halide, in this instance the residual halide being fluorine. If the tantalum layer is deposited on a dielectric-covered substrate, such as a silicon oxide layer on a silicon base, by plasma-enhanced CVD using tantalum pentafluoride, residual fluorine is trapped with an enhanced concentration near the tantalum/dielectric interface. Under certain circumstances, the peak concentration of residual fluorine can approach 5 atomic percent near the tantalum/oxide interface. Halide atoms are expected to be present at similar levels near the metal/dielectric interface between transition metal-based layers deposited by CVD using a transition metal halide and an underlying dielectric.
The elevated concentration of halogen atoms present at the metal/dielectric interface has been found to correlate with a significantly reduced adhesion of the transition metal-based layer to the underlying dielectric. Halogen atoms significantly disrupt the atomic bonding at the interface between the transition metal-based layer and the dielectric so that the transition metal-based layer, and any overlying layers, are more likely to delaminate from the surface of the dielectric.
There is thus a need for a CVD method that will prevent interfacial halogen atoms from altering the adhesion of a transition metal-based layer deposited by a CVD process on a dielectric-covered substrate.
The present invention provides a method of depositing a transition metal-based layer onto a dielectric-covered semiconductor substrate, wherein the layer of transition metal-based material has a significantly enhanced adhesion to the dielectric. To this end and in accordance with the principles of the present invention, the substrate is heated to a predetermined temperature and the dielectric surface of the substrate is exposed to a gas atmosphere comprising a nitrogen-containing process gas for a predetermined time. The nitrogen may be incorporated by exposing a heated substrate to a plasma generated from the nitrogen-containing process gas or by a non-plasma heat treatment in the gas atmosphere of the nitrogen-containing process gas. As a result of the pretreatment, the dielectric incorporates a quantity of nitrogen at its surface. The pretreatment may further include hydrogen in the gas atmosphere to incorporate a quantity of hydrogen into the dielectric surface. Following the thermal or plasma pretreatment, a layer of a transition metal-based material is deposited by a CVD process, such as plasma-enhanced CVD or thermal CVD, onto the surface of the dielectric via a chemical reaction between a transition metal halide vapor-phase reactant and a reducing gas.
After the deposition, the nitrogen from the pretreatment remains positioned near the metal/dielectric interface. The presence of the nitrogen reduces the effect of by-product halogen atoms from the CVD chemical reaction upon the adhesion of the transition metal-based material to the surface of the dielectric. Where hydrogen is also positioned near the metal/dielectric interface, it too reduces the effect of the by-product halogen atoms. In certain embodiments, the layer of transition metal-based metal may comprise either tantalum or tantalum nitride formed by either thermal CVD or plasma-enhanced CVD utilizing a tantalum halide vapor-phase reactant, such as tantalum pentafluoride.
In one embodiment, the nitrogen is incorporated onto the surface of the dielectric-covered semiconductor substrate in the form of a thin layer of a transition metal nitride. The thickness of the thin layer of transition metal nitride may range from about 0.5 nm to about 2.5 nm. The transition metal can originate from residual transition metal halide adsorbed on internal surfaces of the CVD reactor or can be intentionally introduced into the CVD reactor along with or before the nitrogen-containing process gas.